1. Field of the Invention
The present invention relates to a substrate cover, a charged particle beam writing apparatus, and a charged particle beam writing method. More particularly, for example, the present invention relates to an earth or “grounding” system of a substrate on which a pattern is written using electron beams, a writing apparatus, and a writing method.
2. Description of the Related Art
Microlithography technology which forwards miniaturization of semiconductor devices is extremely important, because only this process performs forming a pattern in semiconductor manufacturing processes. In recent years, with an increase in high integration and large capacity of large-scale integrated circuits (LSI), a circuit line width required for semiconductor elements is becoming narrower and narrower. In order to form a desired circuit pattern on these semiconductor devices, a master pattern (also called a reticle or a mask) with high precision is required. Then, since the electron beam technology for writing or “drawing” a pattern has excellent resolution intrinsically, it is used for manufacturing such high precision master patterns.
FIG. 12 shows a schematic diagram for explaining operations of a conventional variable-shaped electron beam writing apparatus. The variable-shaped electron beam (EB) pattern writing apparatus operates as follows: As shown in the figure, the pattern writing apparatus includes two aperture plates. A first or “upper” aperture plate 410 has an opening or “hole” 411 in the shape of a rectangle for shaping an electron beam 330. This shape of the rectangular opening may also be a square, a rhombus, a rhomboid, etc. A second or “lower” aperture plate 420 has a variable-shaped opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired rectangle. The electron beam 330 that left a charged particle source 430 and has passed through the opening 411 is deflected by a deflector. Then, the electron beam 330 passes through a part of the variable-shaped opening 421 of the second aperture plate 420, and irradiates a target workpiece 340 mounted on a stage that is continuously moving in a predetermined direction (e.g. X-axis direction). In other words, a rectangular shape capable of passing through both of the opening 411 and the variable-shaped opening 421 is written in a pattern writing region of the target workpiece 340 mounted on the stage. This method of writing or “forming” a given variable shape by letting beams pass through both of the opening 411 and the variable-shaped opening 421 is called a variable shaped beam system.
Generally, a target workpiece, such as a mask substrate, is fixed to a stage by a member of a clamping mechanism, for example, on the stage in an electron beam pattern writing apparatus. When a pattern is written on the target workpiece, such as a mask substrate, by the electron beam pattern writing apparatus, an electrical conducting material comprising a layer formed on the surface of the target workpiece, such as a shading film of chromium (Cr), will be charged. If the writing is performed in such a state, the problem arises that the orbit of the electron beam irradiating for writing is bent under the influence of the electrified charge, thereby becoming impossible to write at a desired position. Alternatively, the problem arises that the electron beam becomes blurred. Then, usually, earthing (or “ground connection”) is made for the charged layer. In the conventional earthing system, some contact points are allocated on the target workpiece to couple or “connect” the target workpiece charged to ground potential. However, not only the shading film layer is charged. Since the side of the target workpiece is irradiated by a part of the electron beam, a glass substrate etc. exposed on the surface of the side of the target workpiece is also charged.
As to the target workpiece, such as a mask substrate, to be written by the electron beam pattern writing apparatus, it is originally a mask blank where nothing is written. Conventionally, when writing a pattern on such a substrate, an alignment mark is provided on the stage, and alignment for writing is performed by estimating the position of the mask substrate based on the alignment mark position. However, this method is premised on that the relative position between the mask substrate and the stage does not shift. Therefore, even if the mask substrate shifts on the stage, an immediate coping cannot be performed. The position displacement can be checked only by inspecting a finished pattern after the mask substrate has been written and the processing of developing, etching, etc. has been performed.
As to the position alignment method of the mask substrate in the electron beam pattern writing, the technique is disclosed that an alignment mark is beforehand formed on the mask substrate and highly precise alignment is performed between writing the first layer and writing the second layer (refer to, e.g., Japanese Unexamined Patent Publication No. 5-158218 (JP-A-5-158218)). However, in this technique, there is a problem that since the alignment mark needs to be formed on the mask substrate beforehand before writing the pattern, the number of steps is increased because the steps of writing an alignment mark on the mask substrate, developing and etching thereof are added.
As mentioned above, in the electron beam pattern writing apparatus, it is desired to eliminate the influence of the electrified charge of the substrate side upon the orbit of the electron beam irradiating the substrate. Moreover, it is desired to grasp a highly precise position of the substrate even on which the mark is not specially formed.